<p><b>Abstract</b>—The IBM RS/6000 SP system is one of the most cost-effective commercially available high performance machines. IBM RS/6000 SP systems support the Message Passing Interface standard (MPI) and LAPI. LAPI is a low level, reliable, and efficient one-sided communication API library implemented on IBM RS/6000 SP systems. This paper explains how the high performance of the LAPI library has been exploited in order to implement the MPI standard more efficiently than the existing MPI. It describes how to avoid unnecessary data copies at both the sending and receiving sides for such an implementation. The resolution of problems arising from the mismatches between the requirements of the MPI standard and the features of LAPI is discussed. As a result of this exercise, certain enhancements to LAPI are identified to enable an efficient implementation of MPI on LAPI. The performance of the new implementation of MPI is compared with that of the underlying LAPI itself. The latency (in polling and interrupt modes) and bandwidth of our new implementation is compared with that of the native MPI implementation on RS/6000 SP systems. The results indicate that the MPI implementation on LAPI performs comparably to or better than the original MPI implementation in most cases. Improvements of up to 17.3 percent in polling mode latency, 35.8 percent in interrupt mode latency, and 20.9 percent in bandwidth are obtained for certain message sizes. The implementation of MPI on top of LAPI also outperforms the native MPI implementation for the NAS Parallel Benchmarks.</p>